What "fat adaptation" means
Everyone burns a mixture of carbohydrate and fat during exercise. The more intense the activity, the larger the slice that comes from carbohydrate, because fat simply cannot be oxidized fast enough to sustain high efforts. The body stores about 500 grams of glycogen in muscle and 100 grams in the liver, a limited reserve. Fat, even in lean athletes, holds energy for many hours.
The idea behind low-carb (low carbohydrate) and ketogenic (very low carbohydrate, high fat, to the point of producing ketone bodies) diets is precisely this: restricting carbohydrate for weeks to force the organism to become better at using fat as fuel. The technical term is fat adaptation. The promise is seductive for long races, because whoever depends less on glycogen, in theory, does not "hit the wall" when it runs out.
The adaptation works, but has a price
The biggest test of this premise came from Jeff Volek's laboratory. In the FASTER study, elite ultramarathoners fat-adapted for months reached fat oxidation rates far higher than athletes on a carbohydrate-rich diet. The capacity to burn fat reached values never before recorded in humans.
In other words, the metabolic adaptation is real and measurable. The turning point is that oxidizing more fat does not automatically mean running faster. Here enters the work of Louise Burke, from the Australian Institute of Sport.
Elite endurance: where the evidence raises the alert
Burke put elite race walkers on three diets for three weeks: high carbohydrate, periodized carbohydrate and ketogenic. Everyone trained the same. The ketogenic group did in fact get better at burning fat, but paid dearly in economy, which is the oxygen cost of holding a given speed.
The evidence's reading is direct: using fat as fuel is less efficient from the oxygen standpoint. For the amateur athlete who just wants to finish the distance, that may not weigh. For those disputing seconds, the worsening in economy tends to outweigh any benefit of the larger fat reserve.
High intensity and power: the Achilles heel
Explosive efforts depend on glycolysis and readily available muscle glycogen. Sprints, repeated reps, weightlifting and the final kick of a race demand fast energy that fat does not deliver. The ketogenic diet, by emptying glycogen, messes precisely with this system.
The literature here is more mixed than in endurance. Some studies show a drop in high intensity work, while others found no harm in isolated sprint tests after weeks of adaptation. Even so, the field's official position is cautious when the sport requires repeating maximal efforts.
Pros, cons and the way out of periodization
Putting it on the scale, the evidence points to a clear picture. In favor: greater capacity to oxidize fat, less dependence on refueling carbohydrate in very long races and possible help with fat loss. Against: worsening economy at competition paces, risk of losing lean mass, difficulty at high intensity and an adaptation phase that usually drops performance for weeks.
- Elite endurance competing for time: the studies suggest available carbohydrate keeps winning.
- Very long, low intensity ultra: this is where fat adaptation has the most defenders, though without consensus.
- Strength, speed and intermittent sports: the evidence favors not restricting carbohydrate chronically.
Not by chance, many endurance athletes who explore low carbohydrate end up adopting periodization: they train part of the time with little carbohydrate to stimulate adaptation and reintroduce carbohydrate on hard days and in races. American ultramarathoner Zach Bitter, world record holder for the 100 miles on the track in 2019, is a public example of this flexible approach, varying carbohydrate according to training and competition instead of zeroing it year-round.
What stays
Science does not decree a single winner between carbohydrate and fat: it shows two systems with different purposes. The ketogenic diet proves the body can oxidize fat at rates 2.3 times higher (1.54 g/min versus 0.67 g/min in the FASTER study), but this metabolic efficiency charges a price in oxygen that elite athletes cannot pay, as Louise Burke showed. That is why the most promising model is not choosing a side, but imitating Zach Bitter: using low carbohydrate as a training tool, not a permanent identity, saving carbohydrate for the days when the race actually begins.
